Electromagnetic surveying for hydrocarbon reservoirs

ABSTRACT

An electromagnetic survey method for surveying an area of seafloor that is thought or known to contain a subterranean hydrocarbon reservoir, comprising obtaining a first survey data set with a vertical electric dipole (VED) antenna for generating vertical current loops and a second survey data set with a vertical magnetic dipole (VMD) antenna for generating horizontal current loops. In an alternative embodiment, the VMD antenna is dispensed with and the horizontal electromagnetic field is derived from the naturally occurring magnetotelluric (MT) electromagnetic field. In another alternative embodiment, the VED data is compared with a background geological model instead of VMD or MT data. The invention also relates to a survey apparatus comprising VED and VMD antennae, to planning a survey using this method, and to analysis of survey data taken using this survey method. The first and second survey data sets allow the galvanic contribution to the detector signals collected at a detector to he independently contrasted with the inductive effects. This is important to the success of using electromagnetic surveying for identifying hydrocarbon reserves and distinguishing them from other classes or structure.

BACKGROUND OF THE INVENTION

The invention relates to seafloor electromagnetic surveying for oil andother hydrocarbon reserves.

Determining the response of the sub-surface strata within the earth'scrust to electromagnetic fields is a valuable tool in the field ofgeophysical research. The geological structures associated withthermally, hydrothermally, tectonically or magmatically active regionscan be studied. In addition, electromagnetic surveying, or sounding,techniques can provide valuable insights into the nature, andparticularly the likely hydrocarbon content, of subterranean reservoirsin the context of subterranean oil exploration and surveying.

Seismic techniques are often used during oil exploration to identify theexistence, location and extent of reservoirs in subterranean rockstrata. Whilst seismic surveying is able to identify such structures,the technique is often unable to distinguish between the differentpossible compositions of pore fluids within them, especially for porefluids which have similar mechanical properties. In the field of oilexploration, it is necessary to determine whether a previouslyidentified reservoir contains oil or just aqueous pore fluids. To dothis, an exploratory well is drilled to determine the contents of thereservoir. However, this is an expensive process, and one which providesno guarantee of reward.

Whilst oil-filled and water-filled reservoirs are mechanically similar,they do possess significantly different electrical properties and theseprovide for the possibility of electromagnetic based discriminationtesting. A known technique for electromagnetic probing of subterraneanrock strata is the passive magneto-telluric (MT method. The signalmeasured by a surface-based electromagnetic detector in response toelectromagnetic (EM) fields generated naturally, such as within theearth's upper atmosphere, can provide details about the surroundingsubterranean rock strata. However, for deep-sea surveys, all but thoseMT signals with periods corresponding to several cycles per hour arescreened from the seafloor by the highly conductive seawater: Whilst thelong wavelength signals which do penetrate to the seafloor can be usedfor large scale undersea probing, they do not provide sufficient spatialresolution to examine the electrical properties of the typicallyrelatively small scale subterranean reservoirs. Moreover, since MTsurveying relies primarily on horizontally polarised EM fields, it isintrinsically insensitive to thin resistive layers.

Nonetheless, measurements of electrical resistivity beneath the seafloorhave traditionally played a crucial role in hydrocarbon exploration andreservoir assessment and development. In industry, subterraneanresistivity data have generally been obtained almost exclusively bywire-line logging of wells. There are, though, clear advantages todeveloping non-invasive geophysical methods capable of providing suchinformation from the surface or seafloor. Although inevitably suchmethods would be unable to provide comparable vertical resolution towireline logging, the vast saving in terms of avoiding the costs ofdrilling test wells into structures that do not contain economicallyrecoverable amounts of hydrocarbon would represent a major economicadvantage.

In research fields that are not of commercial interest, geophysicalmethods for mapping subterranean resistivity variations by various formsof EM surveying have been in use for many years [1, 2, 3, 10]. Proposalsfor finding hydrocarbon reservoirs using such EM surveying have alsobeen made [4, 5] and applications to the direct detection ofhydrocarbons using horizontal electric dipole (HED) sources anddetectors have proved successful [6, 7].

To successfully map subterranean resistivity variations in the field ofoil exploration, the orientation of the current flows induced by EMsignals must be carefully considered [6]. The response of seawater andsubterranean strata (which will typically comprise planar horizontallayers) to EM signals is generally very different for horizontally andvertically flowing current components. For horizontally flowing currentcomponents, the coupling between the layers comprising the subterraneanstrata is largely inductive. This means the presence of thin resistivelayers (which are indicative of hydrocarbon reservoirs) do notsignificantly affect the EM fields detected at the surface since thelarge scale current flow pattern is not affected by the thin layer. Onthe other hand, for vertical current flow components, the couplingbetween layers is largely galvanic (i.e. due to the direct transfer ofcharge). In these cases even a thin resistive layer strongly affects theEM fields detected at the surface since the large scale current flowpattern is interrupted by the resistive layer. It is known thereforethat significant vertical components of induced current are required tosatisfactorily perform an EM survey in the field of oil exploration.

However, sole reliance on the sensitivity of vertical current flowcomponents to the presence of a thin resistive layer can lead toambiguities. The effects on detected EM fields arising from the presencea thin resistive layer can be indistinguishable from the effects arisingfrom other realistic large scale subterranean strata configurations. Inorder to resolve these ambiguities, it is known that it is necessary todetermine the response of the subterranean strata to both horizontal(i.e. inductively coupled) and vertical (i.e. vertically coupled)induced current flows [6].

Hence it is important when designing a practical EM survey for detectingburied hydrocarbon reservoirs to distinguish between source and detectorconfigurations in which the coupling between layers is largely inductivedue to horizontal currents (in which case the survey has littlesensitivity to the presence of a thin reservoir) and those in which thecoupling between layers is largely galvanic due to vertical currents (inwhich case blocking of the passage of this current flow by a reservoirleads to a survey which is strongly sensitive to the presence andboundary of hydrocarbon within the reservoir).

FIG. 1 a schematically shows a surface vessel 14 undertaking EMsurveying of a subterranean strata configuration according to apreviously proposed method [6]. The subterranean strata configurationincludes an overburden layer 8, an underburden layer 9 and a hydrocarbonlayer (or reservoir) 12. The surface vessel 14 floats on the surface 2of the seawater 4. A deep-towed submersible vehicle 19 carrying a HEDantenna 21 is attached to the surface vessel 14 by an umbilical cable 16providing an electrical and mechanical connection between the deep-towedsubmersible vehicle 19 and the surface vessel 14. The RED antennabroadcasts a HED EM signal into the seawater 4.

One or more remote detectors 25 are located on the seafloor 6. Eachdetector 25 includes an instrument packages 26, a detector antenna 24, afloatation device 28 and a ballast weight (not shown). In practice, eachdetector antenna 24 will generally comprise an array of antennaelements, for example, a pair of orthogonal dipole antennae elements.The detector antenna 24 measures a signal in response to EM fieldsinduced by the HED antenna in the vicinity of the detector 25. Theinstrument package 26 records the signals for later analysis.

The HED antenna 21 generates both inductive and galvanic current flowmodes with the relative strength of each mode depending on HEDantenna-detector geometry. At detector locations which are broadside tothe HED antenna axis, the inductive mode dominates the response. Atdetector locations which are in-line with the HED antenna axis, thegalvanic mode is stronger [6, 8, 9, 10]. The response at detectorlocations in both the in-line and broadside configurations is governedby a combination of the inductively and galvanically coupled modes andthese tend to work in opposition.

FIG. 1 b shows in plan view an example survey geometry according to thepreviously proposed method in which sixteen detectors 25 are laid out ina square grid on a section of seafloor 6 -above a subterranean reservoir56 having a boundary indicated by a heavy line 58. The orientation ofthe subterranean reservoir is indicated by the cardinal compass points(marked N, E, S and W for North, East, South and West respectively)indicated in the upper right of the figure. To perform a survey, asource starts from location ‘A’ and is towed along a path indicated bythe broken line 60 through location ‘B’ until it reaches location ‘C’which marks the end of the survey path. As is evident, the tow pathfirst covers four parallel paths aligned with the North-South directionto drive over the four “columns” of the detectors. This part of thesurvey path moves from location ‘A’ to ‘B’. Starting from location ‘B’,the survey path then covers four paths aligned with the East-Westdirection which drive over the four “rows” of detectors. Each detectoris thus driven over in two orthogonal directions. The survey iscompleted when the source reaches the location marked ‘C’.

During the towing process, each of the detectors 25 presents severaldifferent orientation geometries with respect to the source. Forexample, when the source is directly above the detector position D1 andon the North-South aligned section of the tow path, the detectors atpositions D5, D6 and D7 are at different ranges in an end-on position,the detectors at positions D2, D3 and D4 are at different ranges in abroadside position and the detector at positions D8 and D9 are midwaybetween. However, when the source later passes over the detectorposition D1 when on the East-West aligned section of the tow path, thedetectors at positions D5, D6 and D7 are now in a broadside position,and the detectors at position D2, D3 and D4 are in an end-on position.Thus, in the course of a survey, and in conjunction with the positionalinformation of the source, data from the detectors can be used toprovide details of the signal transmission through the subterraneanstrata for a comprehensive range of distances and orientations betweensource and detector, each with varying galvanic and inductivecontributions to the signal propagation. In this way a simple continuoustowing of the source can provide a detailed survey which covers theextent of the subterranean reservoir.

This previously proposed method has been demonstrated to provide goodresults in practice. However, some limitations of the method have beenidentified.

Firstly, since the two modes cannot be easily separated there willgenerally be a level of cross-talk between them at a detector and thiscan lead to ambiguities in the results.

Secondly, in order to obtain survey data from both in-line and broadsidegeometries, the HED antenna needs to be re-oriented at each HED antennasurvey location. This requires the surface vessel to make multiplepasses over broadcast locations and can lead to complex and long towpatterns.

Thirdly, a HED antenna based EM survey can only provide the best datapossible at discrete survey locations. This is because of the geometricrequirements of a HED antenna survey which dictate that, at any pointduring the survey, data can only be optimally collected from thosedetectors to which the HED antenna is arranged either in-line orbroadside. At other orientations, separation of the inductively andgalvanically coupled signals becomes more much difficult and data areless reliable. For instance, referring to FIG. 1 b, when the HED antennais at a point on the tow path above the detector marked D1 and on theNorth-South aligned section of the tow path, in-line data can only becollected from the detectors marked D5, D6 and D7, whilst broadside datacan only be collected form the detectors marked D2, D3 and D4. The otherdetectors provide only marginally useful information at this point ofthe survey. Furthermore, if the HED antenna is at the locationidentified by reference numeral 57 in FIG. 1 b which is on a North-Southaligned section of the tow path, in-line data can be collected from thedetectors marked D3, D8, D9 and D10, but broadside data cannot becollected from any of the detectors. Since both broadside and in-linedata are required for optimal analysis, the best data possible with thesquare detector array shown in FIG. 1 b can only be collected frompoints along the tow path directly above the detector locations.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided anelectromagnetic survey method for surveying an area that potentiallycontains a subterranean hydrocarbon reservoir, comprising: obtaining afirst survey data set using a vertical electromagnetic source signal;and obtaining a second survey data set using a horizontalelectromagnetic source signal.

In this case, references to vertical and horizontal indicate that asignificant component, preferably a major component, of the respectivesignals should be aligned with the vertical and horizontal axes. It isnot necessary that the signals are perfectly, or even closely, alignedto the vertical and horizontal axes, although fairly close alignment ispreferred to provide a strong signal and reduce the complexity ofanalysis, for example alignment within ±30° is desirable.

The vertical electromagnetic source signal is preferably provided by avertical electric dipole (VED) source. The VED source can be towed overthe survey area above the seafloor by a surface vessel andinterconnecting umbilical cable.

The horizontal electromagnetic source signal is preferably provided by avertical magnetic dipole (VMD) source. The VMD source can also be towedover the survey area above the seafloor by a surface vessel andinterconnecting umbilical cable.

Most preferably, the VED source and VMD source are part of a singlesubmersible tow vehicle and the collection of the first and secondsurvey data sets is done concurrently, typically by towing the vehicleover the seafloor along a predefined survey path. During surveying, thetow vehicle can be switched at short intervals between activation of theVED source and the VMD source. Alternatively, the VED and VMD sourcescan be operated simultaneously at different frequencies to allow easyseparation of the two signals at the detectors. In either case, thecomplete survey can be carried out with a single tow, i.e. both thefirst and second survey data sets can be obtained concurrently. Incontrast, in previously proposed EM survey methods based on use of asurvey vehicle with a HED antenna, two sets of survey data had to beobtained with different tows over different paths in order to probe theresponse of an area of interest galvanically and inductively. Being ableto collect the first and second survey data sets concurrently is ofgreat practical advantage, since it obviates the need to perform twoseparate tows over different survey paths.

As an alternative to using a VMD source, the horizontal electromagneticsource signal could be provided by magnetotelluric (MT) electromagneticfields, which are naturally occurring fields in the earth's upperatmosphere. In this case, the tow vehicle need only include a VEDsource. (A VED source for determining the resistivity and extent oflarge scale sedimentary structure is known [3, 16]. The known VED sourceextends over the full depth of the seawater in which it operates and isdriven at the quasi-static limit.) If the second survey data set isobtained from MT electromagnetic fields, the data can be obtainedconcurrently with collection of the first survey data set or, morelikely, at a different time.

Since a VED antenna generates electromagnetic field configurations whichare sensitive to the presence of a thin resistive layer, whereas a VMDgenerates electromagnetic field configurations which are sensitive tothe background structure, by operating the VED and VMD antennae atdifferent frequencies, or consecutively at the same frequency, thegalvanically and inductively coupled modes of current transfer can beeasily separated, so avoiding the difficulty of the ‘mixed-mode’ fieldsgenerated in previously proposed survey methods.

Furthermore, the vertical electromagnetic signal induces significantcomponents of electric current which are normal to a surface of a bodyof water in which the electromagnetic survey is performed. This limitsthe deleterious effects of the finite seawater depth compared to thoseseen in previously proposed electromagnetic survey methods. The verticalelectromagnetic signal can therefore be operated in shallower water thanhas previously been possible and allows electromagnetic surveying to beperformed in areas which have not previously been amenable toelectromagnetic surveying.

If the first survey data set is obtained by towing a source of thevertical electromagnetic signal along a tow-path relative to an array ofdetectors within the area of interest, a comprehensive survey can beperformed rapidly. Furthermore, because patterns of current flow inducedby vertical electric dipole (and vertical magnetic dipole)electromagnetic signals are cylindrically symmetric, detector data canbe consistently and reliably collected from all detectors within thearray for all source locations. This has not been possible withpreviously proposed survey methods for which data can only be reliablycollected for specific relative orientations of the source and detector.Furthermore, if the layers comprising the subterranean strata arelargely homogeneous and isotropic, the circular symmetry of the verticalelectromagnetic signal can obviate the need to determine the orientationof the detectors within the area of interest. In cases where the layerscomprising the subterranean strata are not homogeneous and isotropic,knowledge of the detector antenna orientation, for instance measured bya compass, will allow more thorough data analysis to be performed.

According to a second aspect of the invention, there is provided amethod of analysing results from an electromagnetic survey of an areapotentially containing a subterranean hydrocarbon reservoir, comprising:providing a first survey data set obtained using a verticalelectromagnetic source signal; providing a second survey data setobtained using a horizontal electromagnetic source signal; generating afirst normalisation data set specific to the first survey data set;generating a second normalisation data set specific to the second surveydata set; combining the first survey data set and first normalisationdata set to obtain a first results data set that represents a differencebetween the first survey data set and the first normalisation data set;and combining the second survey data set and second normalisation dataset to obtain a second results data set that represents a differencebetween the second survey data set and the second normalisation dataset.

The survey method distinguishes between the presence of structure withinthe subterranean strata which influence galvanically coupled electriccurrents flowing vertically between layers (for example thin resistivelayers which can be indicative of the presence of hydrocarbon), and thepresence of structures which influence inductively coupled currentsflowing horizontally within layers. Differences between the first surveydata set and the first normalisation data set reflect structures in thesubterranean strata which influence both galvanically and inductivelycoupled electric currents, whereas differences between the second surveydata set and the second normalisation data set reflect the presence onlyof structures which influence inductively coupled currents. Comparisonsbetween the first and second data sets therefore allow the two classesof structure to be distinguished and hence the presence of thinresistive layers (such as hydrocarbons) to be inferred. Interpretationof the first and second survey data sets may be improved if they arenormalised to the respective first and second normalisation data sets orfirst and second functions specific to the first and second data setsrespectively.

The vertical electromagnetic source signal is preferably a verticalelectric dipole (VED) electromagnetic signal and the horizontalelectromagnetic source signal is preferably a vertical magnetic dipole(VMD) electromagnetic signal. Alternatively, the horizontalelectromagnetic source signal could be a magnetotelluric (MT)electromagnetic field signal.

The first results data set may represent the difference between thefirst survey data set and the first normalisation data set as a functionof position within the area and the analysis of the first results dataset may include identifying a location of a boundary of the subterraneanhydrocarbon reservoir.

The normalisation data sets or functions may be calculated from a rockformation model or from the first and second survey data sets.

According to a third aspect of the invention there is provided acomputer program product bearing machine readable instructions forimplementing a method of analysing results from an electromagneticsurvey as described above.

According to a fourth aspect of the invention there is provided acomputer apparatus loaded with machine readable instructions forimplementing the method of analysing results from an electromagneticsurvey as described above.

According to a fifth aspect of the invention there is provided a methodof planning an electromagnetic survey of an area that potentiallycontains a subterranean hydrocarbon reservoir, comprising: creating amodel of the area to be surveyed, including a seafloor, a rock formationcontaining a hydrocarbon reservoir beneath the seafloor and a body ofwater above the seafloor; setting values for water depth, depth belowthe seafloor of the hydrocarbon reservoir, and resistivity structure ofthe rock formation; performing a simulation of an electromagnetic surveyin the model of the survey area by calculating first and second surveydata sets for a vertical electromagnetic source signal and a horizontalelectromagnetic source signal.

The vertical electromagnetic source signal is preferably a verticalelectric dipole (VED) electromagnetic signal and the horizontalelectromagnetic source signal is preferably a vertical magnetic dipole(VMD) electromagnetic signal. Alternatively, the horizontalelectromagnetic source signal could be a magnetotelluric (MT)electromagnetic field signal.

Repeated simulations for a number of source-to-detector distances andfrequencies can be performed in order to allow optimum surveyingconditions in terms of source-to-detector distance and frequency of EMsignal for probing the hydrocarbon reservoir to be selected whenperforming an electromagnetic survey. The effects of differing detectorsarray configurations and source tow paths can also be modelled. Themodel for the simulation may include a body of air above the body ofwater, wherein the simulation takes account of signal propagation pathsincluding the body of air when calculating the first and second surveydata sets. The first and second survey data sets may be normalisedrelative to respective first and second normalisation data sets orfunctions specific to the first and second survey data setsrespectively.

According to a sixth aspect of the invention there is provided acomputer program product bearing machine readable instructions forimplementing the method of planning an electromagnetic survey asdescribed above.

According to a seventh aspect of the invention there is provided acomputer apparatus loaded with machine readable instructions forimplementing the method of planning an electromagnetic survey asdescribed above.

According to an eighth aspect of the invention there is provided asubmersible vehicle for subsea electromagnetic surveying, comprising: avertical electric dipole antenna; and a vertical magnetic dipoleantenna.

The vertical and magnetic electric dipole antennae are preferablymounted such that respective dipole axes thereof are aligned.

In the case that the antennae are driven by signals generated by a powersupply in the surface vessel, the submersible vehicle will typicallyinclude a waveform generator such as a cycloconverter operable totransform a high voltage, low current AC drive signal received from anumbilical cable into a low voltage, high current AC drive signal todrive the VMD and VED antennae.

According to a ninth aspect of the invention there is provided a surveyapparatus comprising: a submersible vehicle according to the eighthaspect of the invention; a signal power supply unit for generating ahigh voltage, low current AC signal power supply for the submersiblevehicle; and an umbilical cable releasably connectable at ends thereofto the signal power supply unit and the submersible vehicle. The surveyapparatus can be assembled ready for use by connecting the umbilicalcable to the signal power supply unit, which will typically be locatedon the surface vessel, and the submersible vehicle, which can then bedeployed into the sea.

According to a tenth aspect of the invention there is provided a surfacevessel carrying a survey apparatus according to the ninth aspect of theinvention.

According to an eleventh aspect of the invention there is provided anelectromagnetic survey method for surveying an area that potentiallycontains a subterranean hydrocarbon reservoir, comprising: obtaining afirst survey data set using a first electromagnetic field that driveselectric current in vertical planes; and obtaining a second survey dataset using a second electromagnetic field that drives electric current inhorizontal planes.

According to a twelfth aspect of the invention there is provided amethod of analysing results from an electromagnetic survey of an areathat is thought or known to contain a subterranean hydrocarbonreservoir, comprising: providing a survey data set obtained from avertical electric dipole (VED) source; generating a normalisation dataset specific to the survey data set, wherein the normalisation data setis calculated from a rock formation model; and combining the survey dataset and normalisation data set to obtain a results data set thatrepresents a difference between the survey data set and thenormalisation data set. In this aspect of the invention, thenormalisation data set is of sufficient quality as regards takingaccount of spatial variation in resistivity (in 1, 2 or 3 dimensions) toobviate the need to compare the VED data with VMD or MT data. The rockformation model preferably includes resistivity, and may be derived froma combination of geological data and resistivity data. The geologicaldata can be from seismological surveying and the resistivity data fromwell logging. Other sources, such as neutron data could also be used.

According to a thirteenth aspect of the invention there is provided amethod of planning an electromagnetic survey of an area that is thoughtor known to contain a subterranean hydrocarbon reservoir, comprising:creating a model of the area to be surveyed including a seafloor, a rockformation containing a postulated hydrocarbon reservoir beneath theseafloor, and a body of water above the seafloor; setting values forwater depth, depth below the seafloor of the postulated hydrocarbonreservoir, and resistivity structure of the rock formation; andperforming a simulation of an electromagnetic survey in the model of thesurvey area by obtaining a survey data set from a simulated verticalelectric (VED) dipole source; and adjusting the model to remove thepostulated hydrocarbon reservoir and repeating the simulation to obtaina normalisation data set for comparison with the survey data set.

According to a fourteenth aspect of the invention there is provided amethod of monitoring an area that contains a subterranean hydrocarbonreservoir by electromagnetic surveying, comprising: obtaining a firstVED survey data set of the area; waiting a period of time; obtaining asecond VED survey data set; and combining the first and second VEDsurvey data sets to obtain a results data set that represents adifference between the first and second VED survey data sets, saiddifference being indicative of changes of the reservoir during saidperiod of time.

Optionally, the first and second VED survey data sets can also combinedwith a normalisation data set. Preferably, each of the first and secondVED survey data sets are independently normalised before combining them,typically using the same normalisation data set, although differentnormalisation sets could be used. It would also be possible to normaliseonly one of the two data sets. Another possibility would be to obtainthe results data set before normalisation and then normalise thecombined results data set. In other cases, no normalisation may benecessary. The normalisation data set can be obtained from a survey ofthe area by a vertical magnetic dipole (VMD) source or obtained from asurvey of the area by naturally occurring magnetotelluric (MT)electromagnetic fields or calculated from a rock formation model, forexample.

Typically the period of time between VED surveys will be at least a fewdays and usually of the order of weeks to a few months. For example, theperiod of time between obtaining VED survey data sets for comparison canbe at least a day, a week, or a month.

The method is particularly useful for identifying changes in boundariesof the reservoir during the period of time through analysis of thechanges in the VED survey data, for example to monitor reservoirdepletion.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings.

FIG. 1 a shows in schematic vertical section a surface vesselundertaking an EM survey according to a previously proposed method;

FIG. 1 b is a schematic plan view showing an example survey geometryaccording to the previously proposed method in which sixteen detectorsare laid out on a section of seafloor above a subterranean reservoir;

FIG. 2 a shows in schematic vertical section a surface vesselundertaking an EM survey according to an embodiment of the invention;

FIG. 2 b shows a vertical electric dipole (VED) antenna and a verticalmagnetic dipole (VMD) antenna for use in the EM survey shown in FIG. 2a;

FIG. 3 a schematically shows some instantaneous characteristic electricand magnetic field lines associated with a VED antenna;

FIG. 3 b schematically shows some instantaneous characteristic electricand magnetic field lines associated with a VMD antenna;

FIG. 4 schematically shows in vertical section a model subterraneanstrata configuration;

FIG. 5 a is a graph schematically showing the strength of the verticaland radial components of electric field seen at a detector in responseto a VED EM signal for two different subterranean strata configurations;

FIG. 5 b is a graph schematically showing the phase of the vertical andradial components of electric field seen at a detector in response to aVED EM signal for two different subterranean strata configurations;

FIG. 6 a is a graph schematically showing the strength of the azimuthalcomponent of electric field seen at a detector in response to a VMD EMsignal for two different subterranean strata configurations;

FIG. 6 b is a graph schematically showing the phase of the azimuthalcomponent of electric field,seen at a detector in response to a VMD EMsignal for two different subterranean strata configurations;

FIG. 7 is a graph schematically showing the normalised field strengthsof the vertical, radial and azimuthal components of the electric fieldsshown in FIGS. 5 a and 6 a;

FIG. 8 schematically shows in vertical section another modelsubterranean strata configuration;

FIG. 9 a is a graph schematically showing the strength of the verticaland radial components of electric field seen at a detector in responseto a VED EM signal for two different subterranean strata configurations;

FIG. 9 b is a graph schematically showing the phase of the vertical andradial components of electric field seen at a detector in response to aVED EM signal for two different subterranean strata configurations;

FIG. 10 a is a graph schematically showing the strength of the azimuthalcomponent of electric field seen at a detector in response to a VMD EMsignal for two different subterranean strata configurations;

FIG. 10 b is a graph schematically showing the phase of the azimuthalcomponent of electric field seen at a detector in response to a VMD EMsignal for two different subterranean strata configurations;

FIG. 11 is a schematic plan view showing an arrangement of sixteendetectors on a section of seafloor above a subterranean reservoir, anexample EM source tow path is also shown.

FIG. 12 a is a graph showing the normalised strength of the verticalcomponent of electric field seen at a detector in response to a VMD EMsignal for different depths of seawater;

FIG. 12 b is a graph showing the normalised strength of the radialcomponent of electric field seen at a detector in response to a VMD EMsignal for different depths of seawater;

FIG. 13 a schematically shows in vertical section another modelsubterranean strata configuration;

FIG. 13 b schematically shows in horizontal section the modelsubterranean strata configuration shown in FIG. 13 a; and

FIG. 14 is a graph schematically showing the strength of the verticaland radial components of electric field seen at a detector in responseto a VED EM signal for three different subterranean strataconfigurations.

DETAILED DESCRIPTION

FIG. 2 a of the accompanying drawings schematically shows a surfacevessel 14 undertaking EM surveying of a subterranean strataconfiguration (or rock formation) according to an embodiment of theinvention. Features shown in FIG. 2 a which are functionally similar tofeatures shown in FIG. 1 a are given the same reference numeral butshall be described again for the sake of completeness. The subterraneanstrata configuration includes an overburden layer 8, an underburdenlayer 9 and a hydrocarbon layer (or reservoir) 12. The surface vessel 14floats on the surface 2 of the seawater 4. A deep-towed submersiblevehicle 19 carrying an EM source 18 is attached to the surface vessel 14by an umbilical cable 16 providing a detachable electrical andmechanical connection between the deep-towed submersible vehicle 19 andthe surface vessel 14. The deep-towed vehicle 19 includes an echolocation package 27 which assists in maintaining the EM source 18 at anappropriate height above the seafloor 6. The umbilical cable may be astandard deepwater remotely operated vehicle cable including threehigh-voltage conductors and three optic fibres. The EM source 18includes a vertical electric dipole (VED) antenna 22 for broadcasting afirst electromagnetic source signal in the form of a VED EM sourcesignal (giving rise to VED EM signals within the seawater andsubterranean strata configuration) and a vertical magnetic dipole (VMD)antenna 23 for broadcasting a second electromagnetic source signal inthe form of a VMD EM source signal (giving rise to VMD EM signals withinthe seawater and subterranean strata configuration). The EM source 18receives electrical power from the ship's on-board power supply via theumbilical cable 16.

A signal power supply unit 30 on the surface vessel 14 provides signalpower supplies to a waveform generator in the form of a cycloconverter29 via the umbilical cable 16. In this example, the cycloconverter 29 isin the deep-towed submersible vehicle. The cycloconverter 29 generatessuitable VED and VMD alternating current (AC) drive signals from thesignal power supplies and these are supplied to the VED antenna 22 andVMD antenna 23 within the EM source 18. The VED and VMD AC drive signalsrespectively comprise a first and a second set of frequency components.The AC drive signals drive the VED antenna 22 and VMD antenna 23 tobroadcast the VED and VMD EM signals into the seawater 4. In thisexample, the VED antenna and the VMD antenna are driven (via thecycloconverter) by separate signal power supplies from the signal powersupply unit 30. The separate signal power supplies are carried to thecycloconverter 29 on one of the high voltage conductors within theumbilical cable and are returned together on the remaining conductoracting as a common earth. The signal power supply unit 30 is poweredfrom a standard 3-phase power supply on the surface vessel. In otherexamples, a signal power supply unit may be located within the deeptowedsubmersible and be supplied via the umbilical cable with a standard3-phase power supply on the surface vessel. The signal power supply unitgenerates stable-frequency high-voltage (order 2 kV) AC signal powersupplies which are transformed and switched at the deep-towedsubmersible vehicle by the cycloconverter. The cycloconverter includes atransformer and switching bridge to produce suitable high current VEDand VMD AC drive signals to apply to the VED and VMD antennae. Bydriving the VED and VMD antennae at different frequencies, the VED andVMD EM detector signals can be easily separated. Alternatively only oneof the VED and VMD antennae could be driven at any one time, with arouting switch being used to switch between the two antennae, forexample at regular intervals of a few minutes.

Another design option is to generate independent VED and VMD signalpower supplies from separate signal power supply units housed in thesurface vessel, the signal power supplies being supplied separately downa common umbilical cable. The VED and VMD AC drive signals could then behandled by separate cycloconverters (or other waveform generators) inthe submersible vehicle 19, thereby providing two independent electricalsystems for the two signals.

One or more remote detectors 25 are located on the seafloor 6. Eachdetector 25 includes an instrument package 26, a detector antenna 24, afloatation device 28 and a ballast weight (not shown). The detectorantenna 24 produces a VED detector signal and a VMD detector signal inresponse to the VED and VMD EM signals in the vicinity of the detector25 (the terms VED and VMD detector signal are used to indicate which ofthe VED and VMD EM signals the detector signal corresponds to, and notto reflect the EM field pattern in the vicinity of the detector 25). Theinstrument package 26 records the VED and VMD detector signals for lateranalysis. The detector signals may include multiple components of the EMfield (both magnetic and electric) in the vicinity of the detector. Thedetector antenna 24 in this example includes three orthogonal electricdipole detector antennae, a first one arranged vertically for measuringthe vertical electric field component of the VED and VMD EM signals atthe detector, and second and third orthogonal ones arranged in ahorizontal plane for measuring orthogonal horizontal electric fieldcomponent of the VED and VMD EM signals at the detector. The detectorantenna 24 detects (and the instrument package 26 accordingly records)both the amplitude and phase of the electric field arising from the VEDand VMD EM signals at the detector 25 resolved along these threeorthogonal directions.

FIG. 2 b is a schematic perspective view further detailing the EM source18 and the deep-towed submersible vehicle 19. The deep-towed submersiblevehicle is attached to the umbilical cable 16 by a releasable mechanicalcoupling 35. As noted above, the EM source includes a VED antenna 22 anda VMD antenna 23. The VED antenna is attached to the deep-towedsubmersible vehicle by a releasable mechanical coupling 31. The VMDantenna is mounted on a support frame 36 which is attached to a lowerend of the VED antenna

The VED antenna includes an upper VED antenna electrode 32 and a lowerVED antenna electrode 34 separated by a VED antenna main body 33. TheVED AC drive signal from the cycloconverter is coupled to the VEDantenna electrodes 32, 34 via cabling within the deep-towed submersiblevehicle and VED antenna main body 33. The conducting seawater 4 presentsan unscreened return path for the electrical current comprising the VEDAC drive signal and so provides the VED EM source signal. In thisexample, the VED antenna 22 has a length (i.e. distance between theupper and lower antenna electrodes) of 100 m and is supported such thatthe lower electrode is maintained by the echo location package 27 at aheight of 10 m above the seafloor.

The VMD antenna 23 is a loop antenna and is coupled to the VMD AC drivesignal from the cycloconverter via cabling within the deep-towedsubmersible vehicle, the VED antenna main body 33 and a length of VMDantenna connection cable 37. The VMD antenna may comprise a single loop,but in this example, several conductor turns are employed to provide alarger magnetic dipole moment for a given supply current and loopdiameter.

Whilst not shown in this example, a faired cowling may be placed aroundthe VED and/or VMD antenna to reduce drag and help maintain the verticalalignment of the upper and lower electrodes. Stabiliser fins, again notshown in this example, may also be employed to assist in maintaining thestability and orientation of the VED and/or VMD antennae during towing.A short streamer or drogue may also be attached to the VMD to assisttowing stability. At low towing speeds (for example <˜0.8 ms⁻¹ (<˜1.5kts) over the ground) the umbilical 16 between the surface vessel 14 andthe mechanical coupling 35 shown in FIG. 2 b is approximately verticalclose to the EM source 18. Nonetheless, standard ultra-short base lineacoustic navigation and location methods can be used to determine theposition of upper and lower ends of the VED antenna to better than a fewmeters and any deviation of the VED antenna from vertical can thereforebe accounted for in subsequent data processing. It is therefore notessential that the VED antenna induces only vertical current flow loops,although the VED detector signal will be strongest, and the signal datamore easily interpretable, if a major component of the current flowinduced by the VED antenna is in a vertical plane.

FIG. 3 a is a schematic diagram showing some instantaneouscharacteristic electric and magnetic field lines associated with the VEDantenna when driven by the VED AC drive signal from the cycloconverter.The electric field is schematically shown by solid lines and markedE^(VED), and the magnetic field is schematically shown by broken linesand marked B^(VED). The electric E^(VED) and magnetic B^(VED) fields areboth cylindrically symmetric about the long-axis of the VED antenna 22.The electric field E^(VED) drives current (i.e. has components) onlyalong radial and vertical directions in a cylindrical co-ordinate systemcentred on and aligned with the dipole antenna 22, there is no azimuthalcomponent of the electric field E^(VED). The magnetic field B^(VED)associated with the VED antenna, on the other hand, has only azimuthalcomponents. Whilst not shown for simplicity, the magnetic field B^(VED)is not confined to the horizontal plane containing the centre of the VEDantenna 22. An electromagnetic field configuration of the form shown inFIG. 3 a is often referred to as a toroidal magnetic (TM) fieldconfiguration.

FIG. 3 b is a schematic diagram showing some instantaneouscharacteristic electric and magnetic field lines associated with the VMDantenna 23 when driven by the VMD AC drive signal from thecycloconverter. The electric field is schematically shown by solid linesand marked E^(VMD), and the magnetic field is schematically shown bybroken lines and marked B^(VMD). The electric E^(VMD) and magneticB^(VMD) fields are both cylindrically symmetric about a vector centredon and perpendicular to the plane of the VMD antenna loop. The electricfield E^(VMD) drives current (i.e. has components) only along azimuthaldirections in a cylindrical co-ordinate system centred on andperpendicular to the VED antenna loop, there are no radial or verticalcomponents of the electric field E^(VMD), accordingly the VMD EM sourcesignal induces current flow in horizontal planes. The magnetic fieldB^(VMD) associated with the VMD antenna, on the other hand, has bothradial and vertical components, but no azimuthal components. Whilst notshown for simplicity, the electric field E^(VMD) is not confined to thehorizontal plane containing VMD antenna 23. An electromagnetic fieldconfiguration of the form shown in FIG. 3 b is often referred to as atoroidal electric (TE) field configuration.

Since a VED generates only TM mode fields, which are sensitive to thepresence of a thin resistive layer, whereas a VMD generates only TE modefields, which are sensitive to the background structure, by operatingthe sources at two slightly different frequencies, or consecutively atthe same frequency, the two modes can be easily separated, so avoidingthe difficulty of the ‘mixed-mode’ fields generated in previouslyproposed survey methods.

Whilst the electric field configuration for the VED antenna shown inFIG. 3 a includes horizontal radial components in addition to verticalcomponents, it is the vertical components of electric field (which arenot present in the electric field configuration for the VMD antennashown in FIG. 3 b) which play a crucial role in the galvanic couplingmechanism. Accordingly, the electric field configuration for the VEDantenna is sometimes referred to as a vertical electric field, althoughit will be understood that the electric field comprises vertical loopswhich include non-zero radial horizontal components.

During an EM survey, the VED and VMD EM source signals generate VED andVMD EM signals that propagate outwards both into the seawater 4 anddownwards through the seafloor 6 and into the subterranean strata 8, 9,12 comprising the subterranean strata configuration. In each case, atpractical frequencies for this method and given the typical resistivityof the media 4, 8, 9, 12, propagation occurs by diffusion of EM fields.The rate of decay in amplitude and the phase shift of the VED and VMD EMsignals are controlled both by geometric spreading and by skin deptheffects. Because in general the subterranean strata 8, 9, 12 are moreresistive than the seawater 4, skin depths in the subterranean strata 8,9, 12 are longer. As a result, the detector signals seen at the seafloor6 by the detector 25 (at EM source-detector separations R of greaterthan a few hundred metres) are dominated by the components of the VEDand VMD EM signals which have propagated downwards through the seafloor6, along within the subterranean strata 8, 9, 12, and back up to thedetector 25.

At the end of the EM surveying experiment, a remotely operable releasesystem allows the instrument package 26 to be detached from a ballastweight (not shown) so that an in-built flotation device 28 can carry theinstrument package 26 to the surface 2 for recovery and retrieval ofdata for analysis and interpretation. Both the amplitude and the phaseof the detector signals recorded during a survey depend on theresistivity structure of the subterranean strata 8, 9, 12—and so, inprincipal, a survey consisting of many source (transmitter) and detector(receiver) locations can provide a multi-dimensional image, bygeophysical inversion, of subterranean resistivity.

The technique described here exploits the large resistivity contrastthat exists between a hydrocarbon reservoir (typically tens of Ωm orhigher) and the over- and under-lying sediments (typically ˜2 Ωm orless). Such a contrast has a detectable influence on controlled sourceelectromagnetic (CSEM) data collected at the seafloor 6 above thehydrocarbon reservoir 2. The effect of the reservoir is most detectablein CSEM data at an appropriate frequency, and if the horizontal rangefrom the EM source 18 to the detector antenna 24 is of the order of 2 to5 times the depth of burial of the reservoir 12 in typical situations.

The following text describes specific EM source configurations,geometries and data reduction approaches that allow the effect of ahydrocarbon reservoir on the outcome of a controlled sourceelectromagnetic survey to be detected and analysed in practice.

A Specific Approach to Combining Contrasting Dipole Sources WhenProspecting for Hydrocarbon Filled Reservoirs

The survey method shown in FIG. 2 a exploits the diverse properties ofelectromagnetic induction outlined above to provide a first survey dataset (derived from the VED detector signals) which is sensitive to thepresence of thin resistive layers (exploiting the largely galvaniceffects associated with vertical components of the induced current flow)while simultaneously obtaining a second survey data set (derived fromthe VMD detector signals) which is more sensitive to the backgroundsedimentary structure, but relatively insensitive to a thin hydrocarbonlayer or reservoir (exploiting the dominantly inductive effectsassociated with horizontal components of the induced current flow). Bycollecting both data types potential ambiguities in interpretation canbe avoided.

Unlike a single BED source (such as shown in FIG. I a and discussedabove) which excites both the inductive and galvanic current transfermodes simultaneously, the EM source 18 described above excites each modeindependently. The VED 22 generates vertical current loops, and excitesthe galvanically coupled mode whilst the VMD 23 generates horizontalcurrent loops and excites the inductively coupled mode. Since the VED 22and VMD 23 can be driven independently (either at different times fromeach other or simultaneously) the effects of the two current transfermodes (i.e. galvanic and inductive) can be easily separated during dataanalysis.

FIG. 4 shows in schematic vertical section a first, or hydrocarbon,model subterranean strata configuration which is used to examine thediffering responses of the subterranean strata to the VED and VMD EMsignals. A VED antenna 22, a VMD antenna 23 and a detector antenna 24are shown, other components of the survey equipment previously seen inFIG. 2 a are not included for simplicity, but will be understood fromthe description above. In the hydrocarbon model subterranean strataconfiguration shown in FIG. 4, a section of seafloor 6 lies beneath aninfinite depth of seawater 4 which has resistivity 0.3 Ωm. Thesubterranean strata beneath the seafloor 6 comprise a 1 km thickoverburden layer 8, a 100 m thick hydrocarbon, layer 12, and aninfinitely thick underburden layer 9. The overburden layer 8 and theunderburden layer 9 have relatively low resistivities, in this example 1Ωm, due to aqueous saturation of pore spaces. The hydrocarbon reservoir12 has a relatively high resistivity, in this example 100 Ωm, due to thepresence of the poorly-conducting hydrocarbon occupying much of the porespaces.

FIG. 5 a is a graph schematically showing the logarithm of the modelledradial and vertical electric field component amplitudes, Log₁₀(E), seenby a detector in response to a VED EM source signal as a function ofseparation, or range, R, between the VED antenna and the detector. Inthis example, the VED AC drive signal is a quasi-square wave at afrequency of 0.25 Hz and the electric fields are calculated per unitsource electric dipole moment. The VED antenna is 100 m long andsupported such that the lower electrode is 10 m above the seafloor. Themodel field strengths are shown both for the hydrocarbon model strataconfiguration shown in FIG. 4 (solid lines) and for a second, ornormalising, model subterranean strata configuration (dashed lines). Thenormalising model subterranean strata configuration is similar to thehydrocarbon model subterranean strata configuration seen in FIG. 4, butwithout the hydrocarbon layer (i.e. a continuous infinite half-space ofresistivity 1 Ωm beneath an innite depth of seawater with resistivity0.3 Ωm). In each case, the curves relating to the vertical and radialcomponents of the detected field are marked as ver and rad respectively.

FIG. 5 b is a graph schematically showing the phase, φ, relative to theVED AC drive signal used to drive the VED antenna, of the modelledradial and vertical electric field components plotted in FIG. 5 a.Again, curves are shown both for the hydrocarbon (solid lines) and thenormalising (dashed lines) model subterranean strata configurations andmarked ver and rad for the vertical and radial electric field componentsin the VEM detector signal as appropriate.

It can be seen from FIG. 5 a that at short ranges (<˜2000 m) themodelled radial electric fields for the hydrocarbon model subterraneanstrata configuration show a reduction in magnitude relative to theresponse of the uniform half-space normalising model subterranean strataconfiguration. This is caused by the interruption of current flow by theresistive hydrocarbon layer. At longer ranges however, the modelledradial electric field strength within the hydrocarbon model subterraneanstrata configuration is enhanced relative to the response of thenormalising model subterranean strata configuration. This effect ischaracteristic in the galvanically coupled mode of a structure in whichthe resistivity first increases and then decreases with depth below theseafloor. The vertical electric field components at the seafloor show asimilar pattern, although the decrease in the magnitude of the fields atshorter ranges is more pronounced and extends to around 6000 m. This isbecause of the dependence of the vertical fields on the verticalcomponent of current flow.

It can be seen from FIG. 5 b that the phase of the detected signals isalso strongly dependent on the presence of the hydrocarbon reservoir.For example, beyond about 4000 m, the rate of change of phase withincreasing separation is significantly lower for both the radial andvertical electric field components in the hydrocarbon model subterraneanstrata configuration (solid lines) compared to the normalising modelsubterranean strata configuration (dashed lines).

FIG. 6 a is a graph schematically showing the logarithm of the modelledazimuthal electric field component amplitude, Log₁₀(E), seen by adetector in response to a VMD EM source signal as a function ofseparation, or range, R, between the VMD antenna and the detector. Inthis example, the VMD AC drive signal is a quasi-square wave at afrequency of 0.25 Hz and the electric fields are calculated per unitsource magnetic dipole moment. The VMD antenna is supported 10 m abovethe seafloor. The modelled field strength is again shown for both thehydrocarbon (solid lines) and the normalising (dashed lines) modelsubterranean strata configurations discussed above.

FIG. 6 b is a graph schematically showing the phase, φ, relative to theVMD AC drive signal used to drive the VMD antenna, of the modelledazimuthal electric field components plotted in FIG. 6 a. Again, curvesare shown for both the hydrocarbon (solid lines) and the normalising(dashed lines) model subterranean strata configurations.

It can be seen from FIGS. 6 a and 6 b that the modelled field strengthand the modelled phase are very similar for both the hydrocarbon and thenormalising model subterranean strata configurations. The hydrocarbonlayer has little effect on either of the magnitude or phase of theazimuthal detector signal seen in response to the VMD EM signal (whichinduces horizontal current flows).

As can be seen in FIGS. 5 a and 6 a, the modelled electric fieldcomponent amplitudes drop rapidly with increasing source-detectorseparation R. In a typical EM survey the detector signal amplitude istherefore likely to vary by several orders of magnitude over a usefulrange of source-detector separations. The phase φ of the detector signalincreases steadily with increasing separation. In order to visualisemore clearly the effects of a subterranean strata configuration on thedetector signal during a survey, it is convenient to normalise the VEDand VMD detector signals to first and second normalisation data setsrespectively. The first and second normalisation data sets may reflectcalculated detector signals for model subterranean strata configurationsor functions specific to the VED and VMD EM source signals respectively.For example, one simple reference model to use would be the normalisingmodel subterranean strata configuration described above. In otherexamples it may be preferred to use different models, for example, onesimilar to the normalising model subterranean strata configuration butincluding the true (rather than infinite) depth of seawater at thesurvey site. If appropriate, and if the relevant a priori information isavailable, a more complex reference model may be used, although it isgenerally desirable to use the simplest reference model that can broadlyrepresent the large scale background properties of the subterraneanstrata.

FIG. 7 is a graph schematically showing the same electric fieldstrengths plotted in FIGS. 5 a and 6 a for the hydrocarbon modelsubterranean strata configuration but after normalisation by thenormalising model subterranean strata configuration. The normalisedfield strengths N are marked VED_(ver) and VED_(rad) for the verticaland radial electric fields seen in response to a VED EM source signal(solid lines in FIG. 5 a) and VMD_(az) for the azimuthal field seen inresponse to a VMD EM source signal (solid line in FIG. 6 a). Thenormalisation is carried out by dividing the electric fields comprisingthe detector signals calculated for the hydrocarbon model subterraneanstrata configuration by those calculated for the normalising modelsubterranean strata configuration with the same source-detectorgeometry. Whilst not shown, in the case of the phase data, normalisationis carried out by subtracting the phase calculated for the normalisingmodel subterranean strata configuration model from that of thehydrocarbon model subterranean strata configuration.

As an alternative, normalisation could be based on the survey dataitself, for example using data collected adjacent to the target orapplying a smoothing algorithm, such as a box-car mean or alow-frequency spatial filter, to the first and second data sets.

As with FIG. 5 a, it can be seen from FIG. 7 that the presence of thehydrocarbon layer leads to increased radial and vertical electric fieldstrengths seen at a detector in response to a VED EM signal beyondaround 4000 m (radial) and 6000 m (vertical). For example, at around8000 m the radial and vertical components are enhanced by factors ofaround 15 and 5 respectively. As with FIG. 6 a, it can also be seen fromFIG. 7 that the presence of the hydrocarbon layer has little effect onthe azimuthal electric field strengths seen by a detector in response toa VMD EM signal. This is apparent since the normalised electric fieldamplitude is close to unity for all source-detector separations.

Even though the VMD detector signals are not strongly affected by thepresence of the hydrocarbon layer, observing the response of asubterranean strata to horizontally induced current flow is an importantpart of the method since, as noted above and explained further below,characterising the background structure in this way helps to resolvepossible ambiguities in the interpretation of VED detector signal dataalone. This is especially useful in areas of exploration where thebackground structure is not well understood.

FIG. 8 shows in schematic vertical section a third, orincreasing-resistivity, model subterranean strata configuration used tohighlight some of the ambiguities which may arise if only VED detectorsignal data are collected. A VED antenna 22, a VMD antenna 23 and adetector antenna 24 are shown. As with the hydrocarbon modelsubterranean strata configuration, the increasing-resistivity modelsubterranean strata configuration shown in FIG. 9 includes a section ofseafloor 6 beneath an infinite depth of seawater 4 which has resistivity0.3 Ωm. In the increasing-resistivity model subterranean strataconfiguration, the strata beneath the seafloor 6 comprise a 500 m thickfirst layer 40 with resistivity 1 Ωm, a 1 km thick second layer 42 withresistivity 5 Ωm and an infinitely thick third layer 44 with resistivity10 Ωm. The increasing-resistivity model subterranean strataconfiguration approximates to a subterranean strata configurationdisplaying a steadily increasing resistivity with depth. This is not anunreasonable feature of submarine sedimentary basins due to theprogressive expulsion of conductive pore fluids with increasing depthsby a rising overburden pressure.

FIG. 9 a is a graph schematically showing the logarithm of the modelledradial and vertical electric field component amplitudes, Log₁₀(E), seenby a detector in response to a VED EM source signal as a function ofseparation, or range, R, between the VED antenna and the detector. Inthis example, the VED AC drive signal is a quasi-square wave at afrequency of 0.25 Hz and the electric fields are calculated per unitsource electric dipole moment. The VED antenna is 100 m long andsupported such that the lower electrode is 10 m above the seafloor. Themodel field strengths are shown both for the hydrocarbon model strataconfiguration shown in FIG. 4 (solid lines) and for theincreasing-resistivity model subterranean strata configuration shown inFIG. 8 (dashed lines). In each case, the curves relating to the verticaland radial components of the detected field are again marked as ver andrad respectively.

FIG. 9 b is a graph schematically showing the phase, φ, relative to theVED AC drive signal used to drive the VED antenna, of the modelledradial and vertical electric field components plotted in FIG. 9 a.Again, curves are shown both for the hydrocarbon (solid lines) and theincreasing-resistivity (dashed lines) model subterranean strataconfigurations and marked ver and rad for the vertical and radialelectric field components of the VED detector signal as appropriate.

It can be seen from FIGS. 9 a and 9 b that the VED detector signals(both radial and vertical) are similar in amplitude and phase for boththe hydrocarbon and the increasing-resistivity model subterranean strataconfigurations. Accordingly, whilst the presence of a hydrocarbon layerwithin an otherwise uniform subterranean strata configuration can beidentified by observing the response of the subterranean strata to a VEDEM source signal (as shown by the curves in FIGS. 5 a and 5 b), it canbe difficult to distinguish between a hydrocarbon layer within anotherwise uniform subterranean strata configuration and anon-hydrocarbon-containing subterranean strata configuration displayingan increasing resistivity with depth such as is associated with somesubmarine sedimentary basins.

FIG. 10 a is a graph schematically showing the logarithm of the modelledazimuthal electric field component amplitude, Log₁₀(E), seen by adetector in response to a VMD EM source signal as a function ofseparation, or range, R, between the VMD antenna and the detector. Inthis example, the VMD AC drive signal is a quasi-square wave at afrequency of 0.25 Hz and the electric fields are calculated per unitsource magnetic dipole moment. The VMD antenna is supported 10 m abovethe seafloor. As with FIG. 9 a, the modelled field strength is shown forboth the hydrocarbon (solid line) and the increasing-resistivity (dashedline) model subterranean strata configurations.

FIG. 10 b is a graph schematically showing the phase, φ, relative to theVMD AC drive signal used to drive the VMD antenna, of the modelledazimuthal electric field components plotted in FIG. 10 a. Again, curvesare shown for both the hydrocarbon (solid line) and theincreasing-resistivity (dashed line) model subterranean strataconfigurations.

It can be seen from FIG. 10 a that the detected azimuthal electric fieldamplitude seen in response to a VMD EM source signal for the hydrocarbonand the increasing-resistivity model subterranean strata configurationsbeyond a source-detector separation R of about 4000 m begin to differ.This allows the ambiguity between the two models in response to VED EMsource signals alone to be resolved.

In summary, the VED detector signals can be used to distinguish betweena structured subterranean strata configuration (e.g. as shown in FIGS. 4and 7) and a non-structured subterranean strata configuration (e.g. theuniform model subterranean strata configuration described above) and theVMD detector signals can be used to distinguish between differentstructured subterranean strata configurations to identify the presenceof a hydrocarbon layer or reservoir.

Whilst in the above example a VMD EM signal is employed to determine theresponse of the subterranean strata configuration to horizontallyinduced currents, similar information on background structure could begathered using marine magnetotelluric (MT) data [12]. As noted above,the ionospheric EM fields used as the source in MT sounding excitepredominantly horizontal current flow in the earth. MT data aretherefore notoriously insensitive to thin resistive layers, however theycan be used to determine the background structure [7]. It is likely thatin many circumstances VMD detector signal data of the type discussedabove will be preferred since marine MT data are rarely collected atfrequencies high enough to resolve structure at the same scale allowedwhen using controlled source VMD EM source signals. Furthermore, theeffect of distant coastlines can often be seen in MT data and this canincrease the complexity of the interpretation even if the localstructure is simple. A further advantage of employing controlled sourceVMD EM source signals in addition to controlled source VED EM sourcesignals is that provided the VMD antenna is driven at a similarfrequency as the VED antenna, the VMD EM signals provide informationabout background structure at the same scale as that resolved with theVED EM signals.

Use of a Mobile Source and Multiple Fixed Detectors

In order to perform a thorough survey over a large area, and to providecurves similar to those shown in FIGS. 5 a, 5 b, 6 a, 6 b, 8 a, 8 b, 9 aand 9 b, controlled source electromagnetic measurements will typicallybe made with many EM source and detector locations. Whilst these couldbe made in parallel using a plurality of EM sources operating atdifferent frequencies and a plurality of detectors, or in series using asingle EM source and a single detector which are repositioned betweenmeasurements, it will generally be more efficient to employ a pluralityof detectors in fixed positions and a single repositionable EM source,as indicated in FIG. 2 a

The EM source 18 shown in FIG. 2 a can require significant power todrive it, of the order tens of kilowatts, or greater for signalsdetectable at ranges of several kilometres. The umbilical cable 16connecting the EM source 18 to the survey vessel 14 supplies this powerand makes it relatively straightforward to make the EM source mobile. Itcan then be towed in an appropriate survey pattern by the surface surveyvessel 14. Since in many situations surveys of this kind are liable totake place over areas of the seafloor 6 where sensitive engineeringinstallations exist or are planned, there are significant advantages tousing a source which does not physically come into contact with theseafloor 6. Provided that the separation between the EM source 18 andthe seafloor 6 is small compared to a skin depth of the investigatingfield in seawater, the survey can still be completed. (Although it isnoted that coupling of the VED EM signal into the subterranean strata isweaker than that of a HED [13]).

FIG. 11 is a schematic plan view showing an example layout of sixteendetectors 25 distributed across a section of seafloor 6 abovesubterranean reservoir 56. The reservoir 56 has a linear extent on theorder of several km and its boundary is indicated by a heavy line 58.The orientation of the subterranean reservoir is indicated by thecardinal compass points (marked N, E, S and W for North, East, South andWest respectively) indicated in the upper right of the figure. In thisexample, the detectors 25 are uniformly distributed in a square-gridpattern so as to approximately cover the reservoir 56. In performing asurvey, an EM source (not shown) starts from the location marked ‘A’ inFIG. 11 and is towed, whilst broadcasting continuously as describedabove, along a path indicated by the broken line 60, the survey iscompleted when the source reaches the location marked ‘B’. VED and VMDdetector signal data are continuously collected by the detectors 25throughout the towing process and the position of the EM source relativeto the network of detectors is also logged.

During the towing process, each of the detectors 25 presents severaldifferent source-detector separations and orientations relative to theEM source. Accordingly, by following the tow path marked, EM surveydetector signal data are collected for many different source-detectorseparations along many different directions. These detector signal datacan be inverted to provide a thorough map of the subterranean strataconfiguration using appropriate geophysical inversion techniques. Inthis way a simple continuous towing of the source can provide a detailedsurvey which covers the extent of the subterranean reservoir 56.

FIG. 1 b shows for comparison an example tow path followed by apreviously proposed survey geometry using a HED antenna in anelectromagnetic survey of the same area as shown in FIG. 11. Features ofFIG. 1 b similar to those shown in FIG. 11 will be understood from theabove. However, in this example, the tow path is longer. Whilst againstating at the location marked ‘A’, the tow path shown in FIG. 1 b isapproximately twice as long, ending at the location marked ‘C’, as thatshown in FIG. 11. When performing an EM survey according to the presentinvention, detector signal data can be collected from all detectors whenthe EM source is at all points along the tow path shown in FIG. 11,irrespective of orientation.

However, the geometric requirements of a HED antenna survey are suchthat at any point during the survey, data can only be reliably collectedfrom those detectors to which the HED antenna is arranged either in-lineor broadside, as described further above. Accordingly, with a HEDantenna survey, not only is a longer tow path required for a given areaof interest but much less data can be usefully collected along thispath. It is also clear that broadside and in-line data which are to beanalysed together (i.e. relating to the same source and detectorlocations) cannot be collected simultaneously.

Although the above example is based on a square detector grid, it willbe understood that a wide variety of detector placements may be used.For example other high symmetry regular grids, such as triangular orrectangular, may be used. In addition irregular grids may be used thathave no high level of symmetry.

In the case of the detectors, there are further advantages in usingstatic devices. Firstly, since detectors suitable for the EM surveyingare generally internally powered and relatively cheap compared to asuitable EM source, a plurality of detectors can easily by arrayedaround an area of interest so as to provide multiple source-detectorranges and positions for a single EM source position as shown in FIG.11. Secondly, the task of the detectors is to measure electric ormagnetic components of the VED and VMD EM signals at the seafloor 6. Intypical applications, the signal-to-noise ratio of the VED and VMDdetector signals is important to the success and resolution of thesurvey, and should be maximised. Moving a detector inevitably generatesnoise, whether the field components comprising the detector signals aremagnetic or electric. In the case of electric fields, any motion of thedetector through the conducting seawater 4 in the presence of earth'sgeomagnetic field generates an electromotive force across the detectorantenna 24. Detector movements will therefore map into spurious detectorsignals in the recorded data. In the case of magnetic field recordings,there are also advantages to having a static detector. Most importantly,if vector rather than scalar magnetometers are used (i.e. measuringindividual directional components of the magnetic field), any variationin the orientation of the detector antenna 24 will again lead tosignificant spurious detector signals, since the magnetic detectingelement will detect changes in the component of the geomagnetic fieldaligned with it. As a consequence of these effects, any translationalmovement of an electric field sensor comprising the detector antenna 24or rotational movement of a magnetic field sensor comprising thedetector antenna 24 will result in contamination of the detector signalsby motionally induced noise.

For these reasons it is desirable to carry out a controlled sourceelectromagnetic survey to investigate or detect the presence of buriedhydrocarbons using a combination of a mobile EM source operated justabove the seafloor 6 and an array of electric and/or magnetic fieldsensors comprising the detector antenna 24 placed statically on theseafloor 6 as indicated in FIG. 11. At the end of an EM survey, theinstrument packages associated with the detectors are recovered using anacoustically actuated release mechanism to separate it from the ballastweight, allowing them to return to the sea surface for recovery,following standard oceanographic and marine geophysical practice.

Source and detector positions can be determined using standard longand/or short and/or ultra-short baseline acoustic navigation systems andprocessed to determine the separation between the source and detector.With traditional HED antennae, the relative orientation of a source anda detector must also be determined since HED detector signals dependcritically on the relative geometry. This can be a substantial source oferror when surveying with HED antennae. However, since the fields ofboth a VED antenna and VMD antenna are cylindrically symmetric aroundthe source (see FIGS. 3 a and 3 b) only the source-detector separationis required meaning that geometric errors are reduced compared to thoseencountered using a traditional HED antenna.

Furthermore, when using a towed HED antenna, both in-line and broadsidegeometries are required in order to unambiguously determine thesubterranean strata resistivity structure. Although in-line data arestraightforward to collect, ensuring the same coverage in the broadsidedata requires significantly more survey time since, as noted above, datacan only be reliable collected with the HED antenna positioned atcertain discrete positions (or survey locations) relative to a detectorarray and the HED antenna must also make multiple passes at each ofthese positions. An advantage of using a VED antenna and a VMD antennais that detector signals resulting form the both the inductively coupledmode and the galvanically coupled mode can be collected continuously andsimultaneously at each detector during towing, for example by drivingthe VED and VMD antennae at different frequencies. In some circumstances(for instance where the subterranean strata comprise substantiallyisotropic and homogeneous layers), it can be possible to separate VEDand VMD detector signals collected simultaneously even with VED and VMDantennae being driven at the same frequency. For example, andconsidering only electric field detector signal components in ahorizontal plane, this can be achieved by resolving the detector signalalong radial and azimuthal directions. As noted above, the VED and theVMD antennae give rise to only radial and azimuthal electric fieldsrespectively and so the azimuthal and radial components relateindependently to the VED and VMD EM signals. However, if thesubterranean strata does not comprise substantially isotropic andhomogeneous layers then refraction can lead to mixing of the signalsfrom the VED and VMD antennae.

It is noted that if the subterranean strata does comprise substantiallyisotropic and homogeneous layers, and in the case that the VED and theVMD antennae are driven at different frequencies, the resultant of theelectric field amplitude measured in the two orthogonal horizontaldirections of the detector antenna at each frequency reflects the radialand azimuthal components (and hence response of the subterranean stratato the VED and VMD EM signal respectively) regardless of the orientationof the detector antenna. However, in general the subterranean stratawill not comprise substantially isotropic and homogeneous layers andknowledge of the detector antenna orientation, for instance measured bya compass, will allow more thorough data analysis to be performed.

This means that a 2D section of the subterranean strata can bedetermined with relatively few detectors and EM source tows, so reducingthe time taken, and hence cost, for an exploratory EM survey. If a full3D map of the subterranean strata is required, the relative simplicityof the electric and magnetic field geometries associated with VED andVMD antennae means that an image of a target structure can be built upmuch more straightforwardly than using data from existing HED antennasystems.

It will be understood that whilst the above description describes atowed VED antenna, the method would also be applicable in a fixedinstallation. For example, the method could be used to monitor changes ahydrocarbon reservoir from which hydrocarbon is being drawn. In suchcases it will be appropriate to employ one (or more) VED antenna(e) infixed positions relative to a detector array rather than undertakefrequent towed surveys. The VED antenna(e) could be anchored to theseafloor or suspended from a oil-rig platform, for example. In otherexamples, the VED antenna(e) could be placed in a well or borehole, e.g.geotechnical borehole. The One (or more) VMD antenna(e) in fixedpositions could also be used in conjunction with the VED antenna(e).However, since the subterranean structures to which VMD EM signals aresensitive are unlikely to change with time, the results of an initialVMD EM survey (or MT survey) may be sufficient without needing toreproduce the VMD (or MT) part of the EM surveying. In the case of aproducing oil field, the subterranean structures are likely to be wellknown already from prior geophysical surveys and drilling results.Indeed, prior geophysical and geological information from the oil fieldcan be used to construct a background model without the need to collectVMD (or MT) data at any stage.

The Effects of the Seawater Surface

A further consideration in designing an efficient EM survey (forinstance when determining the range of source-detector separations Rover which measurements can reliably be made) is the effects of theseawater surface on the propagation of the VED and/or VMD EM signals.

In shallow water depths, it is possible for EM signals from an EM sourceto follow a propagation path upwards through the seawater to thesurface; horizontally through the air; and back down through theseawater to the seafloor detector. This ‘airwave’ component contains noinformation about the subterranean resistivity and so preferably shouldnot dominate the detector signal.

When using a HED antenna, the air wave component tends to dominate thedetector signals in shallow water and at long source-detectorseparations. The higher the frequency of a drive signal applied to a HEDantenna, the shorter the offset at which the airwave begins to dominatethe detector signals. The effect of the airwave can be minimised bychoosing appropriate transmission frequencies, and by targeting EMsurveys on prospects in deep water and in which the target is at arelatively shallow depth below the seafloor [6].

The effect of the seawater surface is very different for a VED EM signalthan for the signal from a HED antenna. A VED antenna inducespredominantly vertical current loops in the seawater. Because thevertical component of electric field is discontinuous at the sea surface(i.e. electric currents cannot flow across the interface), instead ofpropagating through the air, the EM energy carried in the EM signal isreflected back into the water. The effect of the seawater surface on aVED EM signal is therefore to introduce a surface-reflected signalcomponent (as opposed to an airwave signal component) into the VEDdetector signal. Because the surface-reflected signal is rapidlyattenuated by the conducting seawater, the effect of the seawatersurface on a VED EM signal is more apparent at shorter source-detectorseparations than at longer source-detector separations.

FIG. 12 a is a graph schematically showing model normalised verticalelectric field component amplitudes, N_(V), seen at a detector inresponse to a VED EM source signal as a function of separation, orrange, R, between the VED antenna and the detector for a number ofdifferent overlying depths of seawater. The vertical electric fieldcomponents are calculated for a uniform isotropic half-spacesubterranean strata configuration with resistivity 1 Ωm, and seawaterwith resistivity 0.3 Ωm, and a depth as indicated on each of the curvesin the figure. In each case the electric field components are normalisedby the modelled electric field components seen at a detector with thenormalising model subterranean strata configuration described above(i.e. a uniform isotropic half-space subterranean strata configurationwith resistivity 1 Ωm and seawater with resistivity 0.3 Ωm and infinitedepth). As before, the VED AC drive signal is a quasi-square wave at afrequency of 0.25 Hz and the electric fields are calculated per unitsource electric dipole moment. The VED antenna is 100 m long andsupported such that the lower electrode is 10 m above the seafloor.

FIG. 12 b is a graph schematically showing model normalised radialelectric field component amplitudes, N_(R), seen at a detector inresponse to a VED EM signal as a function of separation, or range, R,between the VED antenna and the detector for a number of differentoverlying depths of seawater. FIG. 12 b is otherwise similar to and willbe understood from the description of FIG. 12 a above.

It is apparent from FIGS. 12 a and 12 b that the effects of a finitewater depth on VED detector signals are most apparent in shallow waterand at short source-detector separations. Larger values of N_(V) orN_(R) indicate that a larger fraction of the detector signal is due tothe surface-reflected signal component. For example, at adetector-source separation of around 1000 m, and a water depth of 0.6km, the radial component of the electric field is approximately doublethat seen at the same separation with an infinite depth of water (i.e.N_(R)=2), the increase in detector signal being due to thesurface-reflected signal component.

As seen in FIGS. 5 a, 5 b and 7, the presence of a hydrocarbon layer inan otherwise homogeneous subterranean strata configuration becomesapparent at source-detector separations beyond around 4000 m and 6000 mfor the radial and vertical electric field components seen at a detectorin response to a VED EM source signal respectively. However, it can alsobe seen that the effects of finite seawater depth shown in FIGS. 12 aand 12 b are beginning to fall away beyond these detector-sourceseparations (i.e. N_(V) and N_(R) are approaching unity), especially forseawater depths greater than around 0.6 km. This is in contrast to a HEDantenna for which finite seawater depths can strongly affect detectorsignals at source-detector separations similar to those at which thedetector signals are typically most sensitive to a hydrocarbon layer orreservoir. Accordingly, surveys which employ a VED antenna can besatisfactorily performed in shallower water than those employingconventional HED systems.

For the VMD EM signal, the airwave component is present. It is similarto that observed with an HED, in that it dominates at long range andhigh frequency (for a given water depth) [15]. However since theresponse to a VMD is sensitive only to the background structure, and notto the hydrocarbon layer the VMD airwave component does not present amajor problem.

On the Detection of the Edges of a Hydrocarbon Layer by this Technique

The model subterranean strata configurations used above to show how acombination of a VED and a VMD antenna can be used to detect ahydrocarbon layer and distinguish it from a general increase inresistivity with depth have all been 1D models (i.e. horizontal layersof infinite horizontal extent). However, VED and VMD antennae EMsurveying also works well in cases where a hydrocarbon layer comprises areservoir of finite horizontal extent, and can also be used as areliable means of locating the edges of an already identified reservoirstructure.

FIG. 13 a shows in schematic vertical section a fourth, or 3D modelsubterranean strata configuration. A VED antenna 22, a VMD antenna 23and a detector antenna 24 are shown. The 3D model subterranean strataconfiguration includes a section of seafloor 6 beneath an infinite depthof seawater 4 with resistivity 0.3 Ωm. The strata beneath the seafloor 6comprise a finite-extent hydrocarbon layer 50 within an otherwiseuniform background structure 52 of infinite horizontal and semi-infinitevertical extent. The uniform background structure has a resistivity of 2Ωm. The finite-extent hydrocarbon layer has a vertical thickness of 100m and an 8×8 km square extent in a horizontal plane, its upper face is 1km below the seafloor, it has a resistivity of 100 Ωm and its centre isdirectly below the VED and VMD antennae.

FIG. 13 b shows a schematic horizontal section through the centre of thefinite-extent hydrocarbon layer 20 within the 3D model subterraneanstrata configuration shown in FIG. 13 a. The projected position of theVED and VMD antennae are marked by a cross 54.

FIG. 14 is a graph schematically showing the logarithm of the modelledradial and vertical electric field component amplitudes, Log₁₀(E), seenby a detector in response to a VED EM signal as a function ofseparation, or range, R, between the VED antenna and the detector forvarious subterranean strata configurations. The separation between thesource and the detector is made horizontally at the seafloor along adirection within the plane of FIG. 13 a and as denoted by the arrowmarked R in FIG. 13 b. In this example, the VED AC drive signal is aquasi-square wave at a frequency of 0.5 Hz and the electric fields arecalculated per unit source electric dipole moment. The VED antenna ismodelled as a point dipole source and supported 10 m above the seafloor.The curves marked 3D-ver and 3D-rad respectively show the modelledradial and vertical electric field component amplitudes seen in responseto the VED EM signal for the 3D model subterranean strata configurationshown in FIGS. 13 a and 13 b. For comparison, curves similar to thoseshown in FIG. 5 a (i.e. detailing the response of a normalising modelsubterranean strata configuration (dashed lines) and a ID hydrocarbonmodel subterranean strata configuration (solid lines)) are shown, butfor consistency are computed for a 0.5 Hz VED AC drive signal and abackground structure resistivity of 2 Ωm, as opposed to a 0.25 Hz VED ACdrive signal and 1 Ωm background structure resistivity. As in FIG. 5 a,these comparison curves are marked ver and rad for the vertical andradial electric field components, and shown as solid and dashed linesfor the hydrocarbon and normalising model subterranean strataconfigurations respectively.

The effects of the finite extent hydrocarbon layer on an otherwiseuniform background are clear in FIG. 14 from the difference between thecurves marked 3D-ver and 3D-rad and the dashed-line curves marked verand rad. The effects of the finite extent of the finite extenthydrocarbon layer are clear in FIG. 14 from the difference between thecurves marked 3D-ver and 3D-rad and the solid-line curves marked ver andrad When both the source and detector are above the finite extenthydrocarbon layer (i.e. at source-detector separations R<4 km) thecurves representing the 3D model subterranean strata configuration(marked 3D-ver and 3D-rad) closely follow the curves representing the 1Dhydrocarbon model subterranean strata configuration (solid-lines markedver and rad). When the detector is not above the finite extenthydrocarbon layer (i.e. at source-detector separations R>4 km), thecurves representing the 3D model subterranean strata configuration beginto approach those representing the normalising model subterranean strataconfiguration (dashed-lines marked ver and rad).

Signal Requirements for Surveying

The EM surveying methods described above can be readily applied tosurveys for hydrocarbon resources on continental margins.

The required EM source and detector characteristics in terms of power,signal to noise ratios and operating parameters will depend onapplication and can in general be met by existing instrumentaltechnology, as can be seen by comparing the modelled electric fieldamplitude with recent publications from the academic sector. Seafloorelectric field detectors can routinely measure electric field detectorsignals of 10⁻¹¹ V/m [7, 14].

It can be seen from FIGS. 5 a, 9 a and 14 that at source-detectorseparations of around 8000 m, where the modelled VED detector signal isaround 10⁻¹⁵ V/m per unit source electric dipole moment, a detectorsignal of 10⁻¹¹ V/m can be obtained by driving the VED antenna with aVED AC drive signal capable of producing an electric dipole moment ofaround 10,000 Am. For the VED antenna described above, a suitable VED ACdrive signal can be provided by a signal power supply unit andcycloconverter similar to those employed in a deep-towed active sourceinstrument (DASI) currently in use [1, 8].

As can be seen from FIGS. 6 a and 10 a, the order of magnitude of theVMD detector signal electric field strengths are generally lower thanthe VED detector signal electric field strengths. Accordingly, a largersource dipole moment is required to provide detector signals which aredetectable at appropriate source-detector separations. For example, at asource-detector separation of around 8000 m, the modelled VMD detectorsignal is around 10⁻¹⁷ V/m per unit source magnetic dipole moment.Accordingly, to distinguish between different subterranean strataconfigurations, a VMD antenna magnetic dipole moment of 10⁶ Am² isrequired. This could be accomplished, for example, with a 3 m diameterloop VMD antenna comprising 1000 turns being driven by a VMD AC drivesignal capable of providing a current of 150 A. The VMD antenna presentsa substantially inductive load, however, power supplies for driving suchloads are readily available, for instance power supplies currently usedfor electric trains or train motors, or other heavy industrialmachinery, which present a high inductance load could be used.

Alternative Embodiment Based on VED Surveying Only

According to an alternative embodiment, it is sufficient to collect VEDdata only. Instead of comparing the VED survey data set with a VMD (orMT) survey data set after normalisation of each set, it is possible tocompare the VED data only with a suitable normalisation data set. In theabove it has been assumed that the normalisation data sets do not havehigh integrity as regards resistivity profile especially, as willusually be the case when an area is poorly surveyed, for example whenconducting initial exploration. However, in some cases an area will bevery well characterised by previous surveying. For example, in aproducing oilfield there is likely to be a wealth of existing seismicand well-log data. In this case, a background rock formation model canbe created from the seismic data and then resistivities assigned to thevarious components in the rock structure using the resistivitiesobtained from well-log data (If directly applicable well-log data is notavailable, it may be possible to estimate resistivity values bycomparison with resistivity data from nearby wells in similar geologicalstructures.) For these reasons, the present embodiment will beespecially suited to applications in existing oilfields, such asmonitoring long term depletion of reserves, as described above under theheading edge detection.

Since the geological/geophysical data, such as seismic and well-logdata, input into the rock formation model in this embodiment will beessentially blind to the presence of hydrocarbon, it may be thought ofas performing an analogous role to the VMD or MT data in the previouslyembodiments.

When monitoring depletion, it may be sufficient to directly compare VEDsurvey data sets taken at different times, e.g. several weeks or monthsapart, without use of normalisation. However, normalisation willgenerally be desirable, either using VMD, MT or rock formation modeldata as discussed above.

Conclusions

The above description shows how controlled source electromagneticsurveying of subterranean strata using a VED antenna and a VMD antenna(or other source for inducing horizontal current flows) can allow thetechnique to be successfully applied to the problem of detectinghydrocarbon layers or reservoirs beneath the seafloor. In particular, wehave identified that:

A high quality survey can be carried out using a combination of a mobileEM source including a VED antenna and a VMD antenna operated just abovethe sea bed, and an array of detectors including electric and/ormagnetic field sensing detector antenna placed statically on theseafloor.

The effects of a finite seawater depth are less severe than seen whenusing a HED antenna This allows a VED antenna to be operated inshallower seawater and in other circumstances where a HED antenna mightnot be useable.

For purposes of data presentation and interpretation it is helpful toemploy detector signal parameters that have been normalised withreference to an appropriate simplified model of the subterranean strata.

In order to resolve the presence of a hydrocarbon layer, and todistinguish the effect of such a layer from other likely subterraneanstrata configurations, it is important to record detector signals seenin response to both inductively (i.e. horizontal current flow) andgalvanically (i.e. vertical current flow) coupled modes of inducedcurrent flow.

3D modelling shows that with an appropriately positioned array ofdetectors, and with an appropriate source tow track, the method canyield valuable information about the limits of the areal extent of anysubterranean hydrocarbon layer, as well as detecting its presence.

Using a VED antenna, the method can also be extended to EM sounding inwells, for use in 4D reservoir monitoring. This would also improve thecoupling of the VED EM signals into the subterranean strata.

It will be understood that whilst the above description has concentratedon dipolar EM sources, similar methods employing other appropriatelyconfigured higher order multipoles, for example quadrupoles or octopolescould also be used.

Finally it will also be understood that the invention is equallyapplicable to surveying of freshwater, for example large lakes, so thatreferences to seafloor, seawater etc. should not be regarded aslimiting.

REFERENCES

-   [1] Sinha, M. C., Patel, P. D., Unsworth, M. J., Owen, T. R. E. &    MacConnack, M. R. G. An active source electromagnetic sounding    system for marine use. Mar. Geophys. Res., 12, 1990, 59-68.-   [2] Evans, R. L., Sinha, M. C., Constable, S. C. & Unsworth, M. J.    On the electrical nature of the axial melt zone at 13°N on the East    Pacific Rise. J Geophys. Res., 99, 1994, 577-588-   [3] Edwards, R. N., Law, K. L., Wolfgram, P. A., Nobes, D. C.,    Bone, M. N., Trigg, D. F. & DeLaurier, J. M., First result of the    MOSES experiment: Sea sediment conductivity and thickness    determination, Bute Inlet, Columbia, bu magnetometric offshore    electrical sounding, Geophyics, 50, 1985, 153-161-   [4] WO 00/13046 A1-   [5] WO 00/57555 A1-   [6] Eidesmo, T., Ellingsrud, S., MacGregor, L. M., Constable, S.,    Sinha, M. C., Johansen, S, Kong, F-N & Westerdahl, H., Sea Bed    Logging (SBL), a new method for remote and direct identification of    hydrocarbon filled layers in deepwater areas, First Break; 20, 2002,    144-152.-   [7] Ellingsrud, S., Sinha, M. C, Constable, S., MacGregor, L. M.,    Eidesmo, T. & Johansen, S., Remote sensing of hydrocarbon layers by    sea-bed logging (SBL): Results from a cruise offshore Angola, The    Leading Edge, submitted 2002.-   [8] MacGregor, L. M. & Sinha, M. C. Use of marine controlled source    electromagnetic sounding for sub-basalt exploration. Geophysical    Prospecting, 48, 2000, 1091-1106.-   [9] WO 02/14906 A1-   [10] MacGregor, L. M., Constable, S. C. & Sinha, M. C. The RAMESSES    experiment III: Controlled source electromagnetic sounding of the    Reykjanes Ridge at 57° 45′ N. Geophysical Journal International,    135, 1998, 773-789.-   [11] MacGregor, L., Sinha, M. & Constable, S. Electrical resistivity    structure of the Valu Fa Ridge, Lau Basin, from marine controlled    source electromagnetic sounding. Geophys. J Int., 146, 217-236,    2001.-   [12] Constable, S., Orange, A. S., Hoverston, G. M. & Morrison, M.    Marine magnetotellurics for petroleum exploration Part I: A    sea-floor equipment system. Geophysics, 63, 1998, 816-825-   [13] Chave, A. D. & Cox, C. S., 1982. Controlled electromagnetic    sources for measuring electrical conductivity beneath the oceans, 1.    Forward Problem and model study, J. Geophys. Res., 87, p 5327-5338-   [14] U.S. Pat. No. 5,770,945-   [15] Coggon, J. H. & Morrison, H. F., Electromagnetic investigation    of the seafloor, Geophysics, 35, 1970 p 476-489-   [16] Edwards, W N., Law, L. K., & DeLaurier, J. M. On measuring the    electrical conductivity of the oceanic crust by a modified    magnetometric resistivity method. J. Geophys. Res., 86, 1981, p    11609-11615

1-30. (canceled)
 31. A method of analyzing results from anelectromagnetic survey of an area that is thought or known to contain asubterranean hydrocarbon reservoir, comprising: providing a first surveydata set obtained from a vertical electric dipole (VED) source;providing a second survey data set obtained from a vertical magneticdipole (VMD) source; generating a first normalization data set specificto the first survey data set; generating a second normalization data setspecific to the second survey data set; combining the first survey dataset and first normalization data set to obtain a first results data setthat represents a difference between the first survey data set and thefirst normalization data set; combining the second survey data set andsecond normalization data set to obtain a second results data set thatrepresents a difference between the second survey data set and thesecond normalization data set; and comparing the first and secondresults data sets to determine if hydrocarbon is present.
 32. A methodof analyzing results from an electromagnetic survey according to claim31, further comprising: normalizing each of the first and second surveydata sets relative to the respective first and second normalization datasets or first and second functions specific to the first and second datasets respectively.
 33. A method of analyzing results from anelectromagnetic survey according to claim 31, wherein the first andsecond normalization data sets or functions are calculated from a rockformation model.
 34. A method of analyzing results from anelectromagnetic survey according to claim 31, wherein the first andsecond normalization data sets or functions are calculated from thefirst and second survey data sets.
 35. A method of analyzing resultsfrom an electromagnetic survey according to claim 31, wherein the firstresults data set represents the difference between the first survey dataset and the first normalization data set as a function of positionwithin the area, and the analysis of the first results data set includesidentifying a location of a boundary of the subterranean hydrocarbonreservoir.
 36. A computer program product bearing machine readableinstructions for implementing a method of analyzing results from anelectromagnetic survey by: providing a first survey data set obtainedfrom a vertical electric dipole (VED) source; providing a second surveydata set obtained from a vertical magnetic dipole (VMD) source;generating a first normalization data set specific to the first surveydata set; generating a second normalization data set specific to thesecond survey data set; combining the first survey data set and firstnormalization data set to obtain a first results data set thatrepresents a difference between the first survey data set and the firstnormalization data set; combining the second survey data set and secondnormalization data set to obtain a second results data set thatrepresents a difference between the second survey data set and thesecond normalization data set; and comparing the first and secondresults data sets to determine if hydrocarbon is present.
 37. A computerapparatus loaded with machine readable instructions for implementing amethod of analyzing results from an electromagnetic survey by: providinga first survey data set obtained from a vertical electric dipole (VED)source; providing a second survey data set obtained from a verticalmagnetic dipole (VMD) source; generating a first normalization data setspecific to the first survey data set; generating a second normalizationdata set specific to the second survey data set; combining the firstsurvey data set and first normalization data set to obtain a firstresults data set that represents a difference between the first surveydata set and the first normalization data set; combining the secondsurvey data set and second normalization data set to obtain a secondresults data set that represents a difference between the second surveydata set and the second normalization data set; and comparing the firstand second results data sets to determine if hydrocarbon is present. 38.A method of analyzing results from an electromagnetic survey of an areathat is thought or known to contain a subterranean hydrocarbonreservoir, comprising: providing a first survey data set obtained from avertical electric dipole (VED) source; providing a second survey dataset obtained from naturally occurring magnetotelluric (MT)electromagnetic fields; generating a first normalization data setspecific to the first survey data set; generating a second normalizationdata set specific to the second survey data set; combining the firstsurvey data set and first normalization data set to obtain a firstresults data set that represents a difference between the first surveydata set and the first normalization data set; combining the secondsurvey data set and second normalization data set to obtain a secondresults data set that represents a difference between the second surveydata set and the second normalization data set; and comparing the firstand second results data sets to determine if hydrocarbon is present. 39.A method of analyzing results from an electromagnetic survey accordingto claim 38, further comprising: normalizing each of the first andsecond survey data sets relative to the respective first and secondnormalization data sets or first and second functions specific to thefirst and second data sets respectively.
 40. A method of analyzingresults from an electromagnetic survey according to claim 38, whereinthe first and second normalization data sets or functions are calculatedfrom a rock formation model.
 41. A method of analyzing results from anelectromagnetic survey according to claim 38, wherein the first andsecond normalization data sets or functions are calculated from thefirst and second survey data sets.
 42. A method of analyzing resultsfrom an electromagnetic survey according to claim 38, wherein the firstresults data set represents the difference between the first survey dataset and the first normalization data set as a function of positionwithin the area, and the analysis of the first results data set includesidentifying a location of a boundary of the subterranean hydrocarbonreservoir.
 43. A computer program product bearing machine readableinstructions for implementing a method of analyzing results from anelectromagnetic survey by: providing a first survey data set obtainedfrom a vertical electric dipole (VED) source; providing a second surveydata set obtained from naturally occurring magnetotelluric (MT)electromagnetic fields; generating a first normalization data setspecific to the first survey data set; generating a second normalizationdata set specific to the second survey data set; combining the firstsurvey data set and first normalization data set to obtain a firstresults data set that represents a difference between the first surveydata set and the first normalization data set; combining the secondsurvey data set and second normalization data set to obtain a secondresults data set that represents a difference between the second surveydata set and the second normalization data set; and comparing the firstand second results data sets to determine if hydrocarbon is present. 44.A computer apparatus loaded with machine readable instructions forimplementing a method of analyzing results from an electromagneticsurvey by: providing a first survey data set obtained from a verticalelectric dipole (VED) source; providing a second survey data setobtained from naturally occurring magnetotelluric (MT) electromagneticfields; generating a first normalization data set specific to the firstsurvey data set; generating a second normalization data set specific tothe second survey data set; combining the first survey data set andfirst normalization data set to obtain a first results data set thatrepresents a difference between the first survey data set and the firstnormalization data set; combining the second survey data set and secondnormalization data set to obtain a second results data set thatrepresents a difference between the second survey data set and thesecond normalization data set; and comparing the first and secondresults data sets to determine if hydrocarbon is present.
 45. Anelectromagnetic survey method for surveying an area that is thought orknown to contain a subterranean hydrocarbon reservoir, comprising:obtaining a first survey data set from a vertical electric dipole (VED)source; obtaining a second survey data set from a vertical magneticdipole (VMD) source; generating a first normalization data set specificto the first survey data set; generating a second normalization data setspecific to the second survey data set; combining the first survey dataset and first normalization data set to obtain a first results data setthat represents a difference between the first survey data set and thefirst normalization data set; combining the second survey data set andsecond normalization data set to obtain a second results data set thatrepresents a difference between the second survey data set and thesecond normalization data set; and comparing the first and secondresults data sets to determine if hydrocarbon is present.
 46. Anelectromagnetic survey method according to claim 45, wherein the VEDsource and the VMD source are mounted on a common submersible vehicle.47. An electromagnetic survey method according to claim 45, wherein thefirst and second survey data sets are obtained together.
 48. Anelectromagnetic survey method according to claim 45, wherein the firstand second survey data sets are obtained separately.
 49. Anelectromagnetic survey method according to claim 45, wherein the VED andVMD sources are operated at different frequencies.
 50. Anelectromagnetic survey method for surveying an area that is thought orknown to contain a subterranean hydrocarbon reservoir, comprising:obtaining a first survey data set from a vertical electric dipole (VED)source; obtaining a second survey data set from naturally occurringmagnetotelluric (MT) electromagnetic fields; generating a firstnormalization data set specific to the first survey data set; generatinga second normalization data set specific to the second survey data set;combining the first survey data set and first normalization data set toobtain a first results data set that represents a difference between thefirst survey data set and the first normalization data set; combiningthe second survey data set and second normalization data set to obtain asecond results data set that represents a difference between the secondsurvey data set and the second normalization data set; and comparing thefirst and second results data sets to determine if hydrocarbon ispresent.
 51. A method of planning an electromagnetic survey of an areathat is thought or known to contain a subterranean hydrocarbonreservoir, comprising: creating a model of the area to be surveyedincluding a seafloor, a rock formation containing a postulatedhydrocarbon reservoir beneath the seafloor, and a body of water abovethe seafloor; setting values for water depth, depth below the seafloorof the postulated hydrocarbon reservoir, and resistivity structure ofthe rock formation; performing a simulation of an electromagnetic surveyin the model of the survey area by obtaining a first survey data setfrom a simulated vertical electric (VED) dipole source and a secondsurvey data set from a simulated vertical magnetic dipole (VMD) source;generating a first normalization data set specific to the first surveydata set; generating a second normalization data set specific to thesecond survey data set; combining the first survey data set and firstnormalization data set to obtain a first results data set thatrepresents a difference between the first survey data set and the firstnormalization data set; combining the second survey data set and secondnormalization data set to obtain a second results data set thatrepresents a difference between the second survey data set and thesecond normalization data set; and comparing the first and secondresults data sets to determine if hydrocarbon is present.
 52. A methodof planning an electromagnetic survey according to claim 51, furthercomprising: repeating the simulation for a number of source-to-detectordistances and frequencies in order to select optimum surveyingconditions in terms of source-to-detector distance for probing thehydrocarbon reservoir.
 53. A method of planning an electromagneticsurvey according to claim 51, wherein the model includes a body of airabove the body of water, and wherein the simulation takes account ofsignal propagation paths including the body of air when calculating thefirst and second survey data sets.
 54. A method of planning anelectromagnetic survey according to claim 51, further comprising:normalizing each of the first and second survey data sets relative torespective first and second normalization data sets or functionsspecific to the first and second survey data sets respectively.
 55. Amethod of planning an electromagnetic survey of an area that is thoughtor known to contain a subterranean hydrocarbon reservoir, comprising:creating a model of the area to be surveyed including a seafloor, a rockformation containing a postulated hydrocarbon reservoir beneath theseafloor, and a body of water above the seafloor; setting values forwater depth, depth below the seafloor of the postulated hydrocarbonreservoir, and resistivity structure of the rock formation; performing asimulation of an electromagnetic survey in the model of the survey areaby obtaining a first survey data set from a simulated vertical electric(VED) dipole source and a second survey data set from simulatedmagnetotelluric (MT) electromagnetic fields; generating a firstnormalization data set specific to the first survey data set; generatinga second normalization data set specific to the second survey data set;combining the first survey data set and first normalization data set toobtain a first results data set that represents a difference between thefirst survey data set and the first normalization data set; combiningthe second survey data set and second normalization data set to obtain asecond results data set that represents a difference between the secondsurvey data set and the second normalization data set; and comparing thefirst and second results data sets to determine if hydrocarbon ispresent.
 56. A method of planning an electromagnetic survey according toclaim 55, further comprising: repeating the simulation for a number ofsource-to-detector distances and frequencies in order to select optimumsurveying conditions in terms of source-to-detector distance for probingthe hydrocarbon reservoir.
 57. A method of planning an electromagneticsurvey according to claim 55, wherein the model includes a body of airabove the body of water, and wherein the simulation takes account ofsignal propagation paths including the body of air when calculating thefirst and second survey data sets.
 58. A method of planning anelectromagnetic survey according to claim 55, further comprising:normalizing each of the first and second survey data sets relative torespective first and second normalization data sets or functionsspecific to the first and second survey data sets respectively.
 59. Acomputer program product bearing machine readable instructions forimplementing the method of planning an electromagnetic survey by:creating a model of the area to be surveyed including a seafloor, a rockformation containing a postulated hydrocarbon reservoir beneath theseafloor, and a body of water above the seafloor; setting values forwater depth, depth below the seafloor of the postulated hydrocarbonreservoir, and resistivity structure of the rock formation; performing asimulation of an electromagnetic survey in the model of the survey areaby obtaining a first survey data set from a simulated vertical electric(VED) dipole source and a second survey data set from a simulatedvertical magnetic dipole (VMD) source; generating a first normalizationdata set specific to the first survey data set; generating a secondnormalization data set specific to the second survey data set; combiningthe first survey data set and first normalization data set to obtain afirst results data set that represents a difference between the firstsurvey data set and the first normalization data set; combining thesecond survey data set and second normalization data set to obtain asecond results data set that represents a difference between the secondsurvey data set and the second normalization data set; and comparing thefirst and second results data sets to determine if hydrocarbon ispresent.
 60. A computer program product bearing machine readableinstructions for implementing a method of planning an electromagneticsurvey by: creating a model of the area to be surveyed including aseafloor, a rock formation containing a postulated hydrocarbon reservoirbeneath the seafloor, and a body of water above the seafloor; settingvalues for water depth, depth below the seafloor of the postulatedhydrocarbon reservoir, and resistivity structure of the rock formation;performing a simulation of an electromagnetic survey in the model of thesurvey area by obtaining a first survey data set from a simulatedvertical electric (VED) dipole source and a second survey data set fromsimulated magnetotelluric (MT) electromagnetic fields; generating afirst normalization data set specific to the first survey data set;generating a second normalization data set specific to the second surveydata set; combining the first survey data set and first normalizationdata set to obtain a first results data set that represents a differencebetween the first survey data set and the first normalization data set;combining the second survey data set and second normalization data setto obtain a second results data set that represents a difference betweenthe second survey data set and the second normalization data set; andcomparing the first and second results data sets to determine ifhydrocarbon is present.
 61. A computer apparatus loaded with machinereadable instructions for implementing a method of planning anelectromagnetic survey by: creating a model of the area to be surveyedincluding a seafloor, a rock formation containing a postulatedhydrocarbon reservoir beneath the seafloor, and a body of water abovethe seafloor; setting values for water depth, depth below the seafloorof the postulated hydrocarbon reservoir, and resistivity structure ofthe rock formation; performing a simulation of an electromagnetic surveyin the model of the survey area by obtaining a first survey data setfrom a simulated vertical electric (VED) dipole source and a secondsurvey data set from a simulated vertical magnetic dipole (VMD) source;generating a first normalization data set specific to the first surveydata set; generating a second normalization data set specific to thesecond survey data set; combining the first survey data set and firstnormalization data set to obtain a first results data set thatrepresents a difference between the first survey data set and the firstnormalization data set; combining the second survey data set and secondnormalization data set to obtain a second results data set thatrepresents a difference between the second survey data set and thesecond normalization data set; and comparing the first and secondresults data sets to determine if hydrocarbon is present.
 62. A computerapparatus loaded with machine readable instructions for implementing amethod of planning an electromagnetic survey by: creating a model of thearea to be surveyed including a seafloor, a rock formation containing apostulated hydrocarbon reservoir beneath the seafloor, and a body ofwater above the seafloor; setting values for water depth, depth belowthe seafloor of the postulated hydrocarbon reservoir, and resistivitystructure of the rock formation; performing a simulation of anelectromagnetic survey in the model of the survey area by obtaining afirst survey data set from a simulated vertical electric (VED) dipolesource and a second survey data set from simulated magnetotelluric (MT)electromagnetic fields; generating a first normalization data setspecific to the first survey data set; generating a second normalizationdata set specific to the second survey data set; combining the firstsurvey data set and first normalization data set to obtain a firstresults data set that represents a difference between the first surveydata set and the first normalization data set; combining the secondsurvey data set and second normalization data set to obtain a secondresults data set that represents a difference between the second surveydata set and the second normalization data set; and comparing the firstand second results data sets to determine if hydrocarbon is present. 63.A submersible vehicle for subsea electromagnetic surveying, comprising:a vertical electric dipole (VED) source; and a vertical magnetic dipole(VMD) source.
 64. A submersible vehicle according to claim 63, whereinthe VED source and the VMD source comprise respective antennae mountedsuch that their dipole axes are aligned.
 65. A submersible vehicleaccording to claim 63, further comprising at least one waveformgenerator operable to transform a high voltage, low current AC drivesignal received from an umbilical cable into a low voltage, high currentAC drive signal to drive the VMD and VED antennae.
 66. A surveyapparatus comprising: a submersible vehicle comprising a verticalelectric dipole (VED) source and a vertical magnetic dipole (VMD)source; a signal power supply unit for generating a high voltage, lowcurrent signal power supply for the submersible vehicle; and anumbilical cable releasably connectable at ends thereof to the signalpower supply unit and the submersible vehicle.
 67. A survey apparatusaccording to claim 66, further comprising a plurality of electromagneticsignal detectors.
 68. A surface vessel carrying a survey apparatusaccording to claim
 66. 69. A static platform carrying a survey apparatusaccording to claim
 66. 70. A well carrying a survey apparatus accordingto claim
 66. 71. A borehole carrying a survey apparatus according toclaim 66.